Each intervertebral disk of the vertebral column is constituted by a central element referred to as the nucleus pulposus that is enclosed in a winding of fibers referred to as the annulus. The disk provides a connection between two vertebral bodies and it controls the flexing, bending, and turning movements of the vertebral column. Such a disk can become damaged under the effect of time, effort, or certain degenerative diseases, and it then becomes flattened and/or ceases to function properly. This can lead to various types of pathology, leading to multiple more or less intense pains and to handicaps of greater or lesser extent.
Treatment for that type of disorder consists in removing the diseased disk and replacing it either with an element that is movable or deformable, or with an element that readily interconnects the two vertebrae concerned.
Several type of prosthesis have been proposed for replacing an intervertebral disk, however they are satisfactory in part only. They do indeed preserve mobility between vertebrae and restore the distance between the vertebrae to a value close to that provided by a healthy disk. However, in providing such mobility, they impose particular dynamics that are not compatible or that are only partially compatible with the natural relative mobility between two vertebrae. In most circumstances, the prosthesis imposes dynamics that are specific thereto, with its center of rotation and its various plane-on-plane guidances, and that inevitably interfere with the natural joint elements that remain between two vertebrae, in particular the posterior articular facets. In this respect it is important to position the prosthesis with care since any inaccuracy in such placement increases the severity of the conflict between the dynamics of the prosthesis and natural dynamics. This non-physiological mobility can lead to undesirable clinical consequences. It is even possible that a component might migrate or that the prosthesis might become dislocated.
Furthermore, most known prostheses are unsuitable for restoring normal cervical or lumbar lordosis. Restoring the distance between vertebrae does not take account of the inclinations needed from one vertebra to the next in the stack of vertebrae in order to attain such natural lordosis, the existence of which is useful for normal biomechanics of the spine as a whole, and in particular of the adjacent levels.
Furthermore, known prostheses are not adapted to absorb shocks. A consequence of this unsuitability, associated with the conflict between the natural dynamics and the dynamics of the joint, can lead to premature wear both in natural elements and in elements of the prosthesis, thereby running the risk of degrading the patient's clinical state.